Porous materials such as activated carbons, zeolites and metal-organic frameworks (MOFs) have been shown to be potentially useful in drug delivery, gas separation, energy storage and catalysis. In these applications, pores or holes with different sizes and geometries play a crucial role in capture and diffusion of guest molecules. Among these materials, hierarchical porous MOFs in which micro- and/or mesoporosity is combined with macroporosity within one structure are particularly beneficial in catalytic applications. These additional large macropores can not only improve the diffusional rate and mass transfer within the MOFs but can also improve molecular accessibility to the microporous cavities which accommodate the important functional groups or active sites of the materials. In this thesis, synthetic methods, characterisation techniques, and catalytic applications of these novel hierarchical porous structures are investigated. A number of advanced techniques were used to characterise these hierarchical MOFs, such as powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and gas sorption analysis. In terms of preparation, all current synthetic methods to obtain hierarchical MOFs with macroporosity were critically reviewed. Then the thesis primarily focusses on post-synthetic acid etching and supercritical CO2 (scCO2) synthesis strategies to synthesise highly crystalline MOFs with micro/meso and macroporous structures. In the etching method, use of phosphoric acid in DMSO and MeOH was systematically investigated to prompt defect formation in prepared HKUST-1, a MOF which is prone to hydrolysis and cannot therefore be etched using standard acid exposure. As a novel finding in this thesis, geometrical interconnected macropores were formed in this MOF using this approach without compromising the bulk crystallinity. Furthermore, this additional porosity was found to be dependent both on pH and etching time. It was shown that the greater the concentration of phosphoric acid (up to pH 2.6 in this study) the more macropores in HKUST-1 were observed. Size-selective acid diffusion was considered as the primary mechanism to form the additional macropores in HKUST-1, which was in agreement with previous work using a similar approach on MIL-100(Fe). With respect to the use of supercritical fluids, as an in situ/direct crystallisation method, scCO2 was introduced to a MOF precursor solution before the nucleation reaction was triggered, enabling a marked reduction in the volumes of organic solvents required for the synthesis. This MOF was also formed rapidly in the presence of scCO2 (after 4 minutes), which is significantly faster than the crystallisation time (24 h) reported for the normal synthesis using DMSO or DMF. It was additionally shown that scCO2 could be used to control the formation of HKUST-1, and importantly, we reported the presence of macropores for the first time in this scCO2-driven MOF. Etching this MOF in scCO2 for over 24 h resulted in additional macroporosity which was not seen in previous reports using compressed CO2. These findings provided a new approach by which macropores with specific performance could be introduced to traditional microporous HKUST-1 MOF. The hierarchical porous HKUST-1 MOF obtained from these two methods was catalytically tested in both gaseous and liquid- phase reactions, showing that introducing macropores into this MOF can have a myriad of advantages in facilitating interactions with the reactants and decreasing the pressure drop, resulting in increased catalytic activity. It was found that the hierarchical pore structure helped to increase the reaction conversion of styrene oxide methanolysis (by ~65% using either HKUST AE and HKUST CO2, at 40 oC in 25 min) and CO oxidation (by 55% using HKUST CO2 at 260 oC). These results suggest different scalable synthetic methods for preparation of materials with hierarchical porous structures, for improved molecular accessibility and diffusion for industrial catalytic applications.
|Date of Award||1 Oct 2019|
- The University of Bristol
|Supervisor||Sean A Davis (Supervisor) & Valeska Ting (Supervisor)|
- Metal-organic framework
- porous material